* Armenia is strategic lynchpin in ex-Soviet South Caucasus (Adds more comments)

By Hasmik Mkrtchyan

YEREVAN, Feb 1 (Reuters) - An assassination attempt on a presidential candidate in Armenia has thrown this month's election into doubt and could threaten stability in the volatile Caucasus region that carries oil and natural gas to Europe.

Paruyr Hayrikyan, an outsider in the Feb. 18 presidential vote, was shot in the shoulder on Thursday night close to his home in the capital Yerevan. Doctors removed the bullet on Friday and said his life was not in danger.

The motive was not immediately clear but the election, in which President Serzh Sarksyan is widely expected to secure a second five-year term, could be postponed for two weeks under the constitution if Hayrikyan is forced to pull out.

"It's obvious that those who were behind this crime aimed to affect the natural flow of elections. This outrageous crime is directed not only against presidential candidate and statesman Hayrikyan but against our security," Sarksyan told reporters after visiting Hayrikyan at the hospital.

"I want to assure everyone that our law-enforcement agencies will do their best to solve this crime."

Stability is vital for the former Soviet republic of 3.2 million to woo investors and boost an economy devastated by a war with neighbouring Azerbaijan in the 1990s and then by the 2008-09 global financial crisis.

The attack will raise fears of a return to the violence that left about 10 people dead during the 2008 presidential election in the landlocked country, Russia's main ally in the turbulent South Caucasus.

"I don't think that this shot was against Mr Hayrikyan," said independent political analyst Alexander Iskanderyan, describing the former dissident as a symbol of independence.

"This shot was aimed against the political development of the country and was meant to destabilise the situation and mar the election."

A few hours after undergoing surgery Hayrikyan said he suspected a foreign secret service, perhaps that of Russia, was behind the attack after his recent statements about Armenia's need to take a pro-Western course.

"I suspect a secret service of a foreign country...I said clearly that there was no alternative for us and we should move closer to Europe," Hayrikyan told reporters at the hospital.

"We've had more harm than advantage from imperialistic Russia... I suggest that the assassination attempt is linked to these statements."

He did not rule out asking for the election, which is to be held in two weeks, to be postponed.

Any sign of instability in the Caucasus is a concern to investors because although Armenia has no pipelines of its own, pipelines carry oil and gas to Europe via Turkey through Azerbaijan, which still has fractious relations with Armenia.

Russia has a military base in Armenia, which is a member of a Moscow-dominated security alliance of ex-Soviet states.

Investors are already worried that violence could break out again over Nagorno-Karabakh, a mountainous enclave inside Azerbaijan that is controlled by ethnic Armenians. A war in the enclave in the early 1990s killed about 30,000 people.

Relations with another neighbour, Turkey, are also fraught because Ankara does not recognise as genocide the killing of Armenians in Ottoman Turkey in World War One. Turkey closed its border with Armenia in solidarity with Turks in Azerbaijan over the Nagorno-Karabakh war.

The Organisation for Security and Cooperation in Europe (OSCE) expressed concern over Thursday's attack.

"This attack is deeply distressing in view of the electoral process already under way, and I urge all stakeholders to refrain from any actions that might further aggravate the situation," said Ambassador Janez Lenarcic, the Director of the OSCE Office for Democratic Institutions and Human Rights.

Hayrikyan, 63, is a Soviet-era dissident who survived Soviet prison and internal exile as well as forced deportation from the Soviet Union to Ethiopia and the United States.

He leads a moderate opposition party, the National Self-determination Union and ran for president in 2003.

Analysts were puzzled about the motive for the attack.

"I see the shooting as both a surprise and not necessarily politically motivated ... It may be the act of a frustrated, disgruntled lone individual," said Richard Giragosian, director of the Regional Studies Centre think tank.

"And the key question - who gains from this act - remains unclear... But it may also reveal the level of discontent lingering just below the surface."

Armenia was isolated and in chaos after the collapse of the Soviet Union in 1991, and things got so bad in the 1990s that people cut down all the trees in Yerevan to use as firewood.

Matters have improved since then, but the 2008-09 global economic crisis set back the recovery and the average nominal monthly salary is still under $300. (Additional reporting and writing by Margarita Antidze; Editing by Hugh Lawson)